1. Field of the Invention
This invention relates to a fault clearing control circuit for a permanent magnet machine. More particularly, fault sensors are provided that identify and localize a fault. Dependent on type and location of fault, proper remedial action is taken.
2. Description of the Related Art
A permanent magnet machine (PMM) has a number of advantages over other types of motors and electrical power generation and/or engine starting systems commonly used on aircraft. The PMM is typically smaller, lighter and more efficient than brushed starter generators and other typical brushless machines used in aircraft applications. The PMM also typically requires less maintenance and exhibits much higher reliability than other commonly used aircraft machines such as starter/generator machines using brushes. The brushed starter/generator also typically exhibits significant reductions in generator capability when required to operate over wide speed ranges as is often required in aircraft applications (e.g., typically 50% to 100% speed range operation is required in small aircraft generator applications). Whereas the PMM configured for use as a permanent magnet alternator (PMA), permanent magnet generator (PMG), and/or permanent magnet starter/generator (PMSG) is typically capable of operating over wide speed ranges with less weight and/or performance penalty. In summary, a PMM, PMA, PMG or PMSG subsystem typically offers significant improvements in reliability, weight, size, performance, and maintenance requirements relative to other commonly used motors, alternators, generators or starter/generators.
A disadvantage of the PMM is the output voltage cannot be controlled by means of adjusting a field excitation (which is a common control technique used with brushed machines), and therefore the output cannot be electronically disabled in the event of a fault. The inability to disable output power during fault mode operation is of particular concern in high reliability applications such as aircraft power systems.
A voltage breakdown in the feeder cable to engine or aircraft structure can result in a hazardous or unsafe condition for typical aircraft generation and distribution systems. However, that type of fault can be relatively easily mitigated by use of contactors, thermal switches or fuses in series with the feeder cable. Another method to mitigate this type of fault is to design the PMA, PMG or PMSG with high series reactance as described in U.S. Pat. No. 7,064,526 by Patterson and U.S. Pat. No. 7,365,521 by Bhargava.
A fault within the PMA, PMG or PMSG cable, load or internal winding may result in a thermal runaway condition that can cause a fire within the machine if that fault is not mitigated in some manner. These faults cannot always be mitigated by use of a series fuse, switch or contactor. Nor is a turn-to-turn fault within a winding of the stator readily mitigated by the method disclosed in U.S. Pat. No. 7,365,521. Furthermore, the extreme response of shutting down the prime mover is not an acceptable alternative for fault mitigation in many aircraft applications.
There remains a need for a fault clearing method for permanent magnet machines that does not have the limitations of the prior art methods.